Flexible polyurethane foams are well recognized articles of commerce. The two most common classifications of flexible polyurethane foams have been conventional and high resilience (HR) as set forth in ASTM D3770; now discontinued. ASTM D 3453-01 currently sets forth specifications for three cushioning grades of flexible urethane cellular materials, which include normal support (NS), high support (HS) and high support-high resilience (HS-HR). The latter designation encompasses the former HR classification. Flexible foams may also be characterized by the process used in production thereof, either molded or free-rise. Free-rise foams are often made in a continuous slabstock process. Molded foams are typically manufactured within an enclosed chamber having the shape of the desired finished article. HS and HS-HR foams are manufactured by both free-rise and molded processes.
HS and HS-HR foams are widely employed in furniture, mattresses, automotive and numerous other applications. HS and HS-HR foams are differentiated from conventional foams by their higher support factor. HS-HR foams are also distinguished by their higher resilience. As set forth in ASTM Standard Specification D 3453-01, HS foams have a minimum support factor of 2.3 whereas HS-HR foams have a minimum support factor of 2.4 and a minimum resilience of 55%. Support factor is the ratio of 65% IFD to 25% IFD and resilience is the Ball Rebound percentage. Measurement specifications for 25% IFD, 65% IFD and Ball Rebound are set forth in ASTM D 3574-01.
In such foams, a high crosslinking density is achieved during foaming through the use of more reactive polyols and typically through the use of a crosslinking agent such as diethanolamine, triethanolamine, glycerine, sorbitol and the like. Higher functionality isocyanates such as polymeric MDI may also be employed in addition to, or instead of, the crosslinking agents. The enhanced crosslinking provides stabilization to the rising foam and obviates the need for a strongly stabilizing silicone surfactant as used in the production of Normal Support (NS) foams. Use of a weak silicone or no silicone surfactant contributes to the formation of foams with higher support factor. However, it is inherent in these foam types that most cell windows remain fully or partially intact at the time of production, thus necessitating a crushing process to enhance the air flow and to achieve the cushioning and property requirements of the end-use application. For very reactive systems, such as those encountered in molded automotive seating, the foams may exhibit a predominantly closed cell structure that requires immediate hot crushing to avoid shrinkage or warpage of the part. Free-rise foams produced by a continuous slabstock process generally do not contain a significant percentage of fully closed cells, and in most cases, need not be crushed until after cooling. Molded and free-rise flexible HS and HS-HR foams made with poly(oxyalkylene) polyols that are polymerized with a double metal cyanide (DMC) alkoxylation catalyst have been found to exhibit increased tightness (U.S. Pat. No. 5,605,939) and can be particularly difficult to crush open.
Processing latitude means the tolerance limits within which it is possible to deviate from a formulation and still maintain commercially acceptable processing and foam property requirements. These limits are typically set on the one end by factors such as poor foam cure, long demold time, instability, and voids, whereas the other end is indicated by foam shrinkage, warpage and inability to crush the foam sufficiently open to achieve good cushioning performance.
Current mechanical methods for cell opening and porosity enhancement of molded foams typically involve compressive crushing, vacuum rupture or time pressure release. Compressive crushing can be accomplished by removing the part from the mold and immediately compressing by hand or between plates, or more commonly by passing through rollers.
Vacuum crushing involves drawing a vacuum on the finished foam causing cell rupture. A commercially attractive method for cell opening is time pressure release (TPR), which requires opening the mold during the curing process to release the internal pressure and reclosing for the duration of the cure time. The sudden release of the internally generated pressure bursts the cell windows, thereby producing a sufficiently open cell foam to avoid shrinkage or warpage (U.S. Pat. Nos. 6,136,876 and 4,579,700). TPR may be supplemented by a later mechanical crushing step to more fully open the cell windows and achieve high air flow.
Free-rise HS and HS-HR foams can be crushed hot if necessary, but more commonly are crushed after cooling because those foams are generally produced without a significant closed cell content and thus are not subject to high shrinkage or warpage during cooling. However, air flow is usually very low until the foams undergo a mechanical crushing process. This often involves passing the large slabstock bun through a multi-stage roller crusher that compresses the bun by progressively larger amounts. Compression by at least 75% (25% of original height) and preferably 90% is generally done to approach the fully crushed air flow. Another approach is to cut the bun into smaller slabs or into the end-use part dimensions and crush these separately. In some cases where the foam is easily crushed, a separate crushing process may be avoided if the foam is opened sufficiently by flexing during the fabrication process or in the end-use application.
Foam crushing can cause a number of problems in the production of molded and free-rise HS and HS-HR foams. Excessively high crushing forces can result in non-useable foam in the intended application if it is not possible to sufficiently open the foam or if the crushing process permanently distorts the part or tears the foam. In the TPR process, there can be a very narrow time window between the time that the foam is insufficiently reacted to open the mold and the time that it is too tight to crush by this process. Foams that do not crush well can result in poor cushioning characteristics, and poor durability due to excessive softening and increased tendencies to take a permanent set. Accordingly, chemical agents to avoid high crushing forces and improve the crushability of HS and HS-HR foams would be welcomed.
U.S. Pat. No. 6,136,876 discloses a polyurethane flexible foam containing an organic polyisocyanate and a polyol in the presence of a catalyst composition, a blowing agent, optionally a silicone surfactant cell stabilizer and an active methylene- or methine (methylidyne) group containing compound which is used as a cell opening agent. The cell opening agents are characterized as weak Brønsted acids. A disadvantage of the approach of the '876 patent is that the weak acids tend to reduce the reactivity of the isocyanate with the active hydrogen components. To compensate, it is often necessary to increase catalyst levels, thereby increasing costs and potentially leading to higher crushing forces. In addition, these cell opening agents may not be chemically stable if blended with other “B” side components as is commonly practiced in molded foam processing. Many HS and HS-HR grades of slabstock and molded foams have very high FTC (Force Required to Crush) values and do not open fully when crushed by standard processes. Therefore, traditional cell opening methods such as the use of less gelling catalysts and less stabilizing silicones have provided only limited success in overcoming this problem. Generally, such cell opening approaches will lower the FTC only with concomitant reduction in other foam processing and foam properties.
The use of liquid polybutadiene as a release agent in the preparation of molded polyurethane and polyurea articles is disclosed in U.S. Pat. No. 5,079,270. This patent is directed to the formation of elastomeric or microcellular elastomeric articles without surface defects that are produced by a RIM process. No reference is made therein to the production of flexible foam with improved crushability and no examples are given in which a low density flexible polyurethane foam is produced. Liquid polybutadiene in an amount of 0.5% to 5% of the total weight of the components of the reaction is disclosed. Based on the compositions cited therein and typical formulations, the 0.5% of the '270 patent would correspond to at least 0.7 parts of liquid polybutadiene per hundred parts of polyol.
U.S. Pat. No. 5,614,566 discloses the use of liquid, higher molecular weight hydrocarbons, such as polybutadiene and polyoctenylene, in the production of rigid foams having an extensively open cell structure. The rigid foams of the '566 patent differ from the flexible foams of the present invention both in the foams' properties and in the components used in the production thereof, paticularly, the high hydroxyl number of the polyol component which, in the '566 patent, is between 100 and 800 (hydroxyl equivalent weights between 70 and 561). The hydroxyl number of flexible foam polyols is typically much less than 100.
Japanese Kokais JP 74-57325 and JP 92-57873 also disclose the use of liquid polybutadiene as a shrinkage inhibitor in rigid foam production.
Thus, the art fails to provide insights into the influence of liquid polybutadiene on the force required to crush flexible foams because rigid foams cannot undergo a crushing process without permanent deformation.
Accordingly, the present invention is directed to a chemical agent for use in high support (HS) and high support-high resilience (HS-HR) flexible foam that will lower the FTC at low use levels and have minimal influence on foam processing, foam odor, and other foam properties.